(1) Field of the Invention
The present invention relates to a transmission electron microscope and a method of observing magnetic phenomena by using its apparatus. To put it in more detail, the present invention relates to a scanning transmission electron microscope apparatus used for observing magnetic phenomena of magnetic substances by means of a transmission electron microscope equipped with a specimen chamber without or with sufficiently weak magnetic field and a method of observing the magnetic phenomena using the apparatus.
(2) Description of the Prior Art
The scanning transmission electron microscope can converge an electron beam into a very small point. Accordingly, by detecting a deflection due to a Lorentz force experienced by the electron beam during the transmission through a specimen (usually a magnetic film), the micro magnetic structure of the magnetic film can be examined with a very high degree of resolution. The transmission intensity of the electron beam can produce an image that can normally be obtained by means of an ordinary scanning transmission electron microscope. Since the angle of deflection of the electron beam is generally smaller than the illumination semi-angle of the incident electron beam hitting the specimen, a detector whose surface is divided into several parts is used for detecting the amount of deflection. Such a detector is called a differential phase contrast detector. By calculating differences among signals from each part of the detector, a signal representing the amount of deflection can be obtained. On the other hand, a signal representing the transmission intensity is produced by computing the sum of the signals from each part of the detector.
In the observation of the magnetic state (or the magnetization image) of a magnetic film, two big problems are encountered. One of the problems is that magnetization in the magnetic thin film is overshadowed and concealed by a stray magnetic field, which is inevitably unseen during the observation. As a result, the contrast of the magnetization cannot be obtained. In a magnetic recording device, for example, magnetizations opposite to each other known as recording bits are written as recorded data by a magnetic recording head onto a magnetic thin film which serves as a recording medium. On the other hand, recorded data is read out from the recording medium by detecting a change in stray magnetic field that leaks from a boundary between magnetized and unmagnetized portions on the thin magnetic film. The configuration of a magnetic recording device is shown in FIG. 8. As shown in the figure, the magnetic recording device comprises a plurality of magnetization regions 81. The magnetization regions 81 have been magnetized in directions indicated by solid-line arrows 82, representing information of recorded bits. Reference numeral 83 denotes stray magnetic fields generated in such directions that the magnetizations 82 of the magnetization regions 81 in the magnetic thin film are neutralized. An electron beam 1 passing through the center of a recording bit 81 is deflected by the magnetization of the magnetization region 81 and the corresponding stray magnetic field by about the same angles of deflection but in opposite angular directions. As a result, the electron beam 1 passes through the magnetization region 81 as if there were almost no deflection effect of the magnetization. As described earlier, the existence of a magnetic field is observed by measuring the amount of deflection. Since almost no deflection is detected, it is also difficult to detect the existence of the magnetic field.
The other problem is caused by contrast originating from crystallites in the magnetic image. With a Lorentz force not existing, an electron beam passing through a magnetic thin film is adjusted by using an imaging lens and a beam shift coil so that the electron beam reaches the center of a differential phase contrast detector. Adjustment is further carried out so that a signal representing differences in intensity among outputs from each part of the differential phase contrast detector becomes zero. With a magnetic field existing in a specimen, on the other hand, the electron beam is deflected by a Lorentz force. Accordingly, the intensities of signals hitting the detecting surfaces of the differential phase contrast detector vary. As a result, the direction and magnitude of the deflection experienced by the electron beam can be known. In general, the magnetic thin film serving as a magnetic recording film, a magnetic thin film of importance to the industry, is a polycrystalline film. The crystallites of a polycrystalline film have crystal orientations different from each other, exposing a variety of different scattering intensities to a passing electron beam. For an electron beam on a grain boundary, a distribution of intensities can thus be observed in a probe examining the passing electron beam. Thus, a signal appears in the differential phase contrast detector even if the electron beam experiences no deflection. In addition, an electrostatic potential is built up on the grain boundary and the gradient of the electrostatic potential also inevitably deflects a passing electron beam as well. Because of factors originating from crystallites, contrast is resulted in due to no magnetic origin. The contrast may, in turn, give rise in the magnetization image. In this case, it will thus be difficult to analyze in detail the state of an image representing the distribution of a magnetic field. As a result, it will also be hard to obtain a magnetization image with a high degree of resolution.
An effective solution to the first problem has not been found so far. In order to solve the second problem, techniques are adopted to reduce contrast caused by crystallites using a frequency filter. The frequency filter is used because spatial frequencies with different magnetic and crystallite structures exist. A typical technique is disclosed in the Journal of Applied Physics 69 (1991) 6078-6083, Mapping Induction Distributions by Transmission Electron Microscopy. With this technique, an electron beam is converged with a diameter of the order of 10 nm and then used for scanning over a specimen being observed. Subsequently, the deflection of the electron beam passing the specimen is observed by means of an eight-division detector. At that time, the spatial-frequency response function of the detector is varied in order to reduce contrast caused by crystallites. The technique of varying the response function is known as a modified differential phase contrast method. As a result, a cross-tie domain-wall of a permalloy thin film with almost no stray magnetic field can be observed at a resolution of the order of the beam diameter and magnetic induction mapping can also be carried out as well. The above conventional modified differential phase contrast method for reducing contrast caused by crystallites has a shortcoming that, since a spatial-frequency filter is employed in the method, spatial-frequency information removed out by the filter is inadvertently lost even if the information is also to be observed. That is to say, it is impossible to reduce only the contrast caused by crystallites due to the fact that the spatial frequencies with different magnetic and crystallite structures do not necessarily exist. In the case of observation of a magnetic recording medium with small interaction among its crystallites, in particular, magnetization ripples in the medium are considered to occur in crystallite units so that the ripples are also reduced as well by the spatial-frequency filter. As a result, fine observation becomes impossible.